Note: My review score is 4.5/5, but as Head-Fi's new platform doesn't allow half points yet, I feel the unit deserves more than 4/5 and have rounded it up, instead of down.

Ah... The elusive Shure SHA900 portable amplifier. Shure as a company needs no introduction, yet this product, while priced like the SE846 earphones, gets only a tiny fraction of its attention.
Well, I think it's time this gets a review on Head-Fi, and shed some light on the great, the good and on what's lacking.

[The amp in action, connected to a laptop.]​

My background and equipment used:
I've been with Shure earphones for about a decade now, starting with the SE210s. Along the way I tried all sort of headphones, earphones, amplifiers and DACs from other brands, and while each had its unique impact, for my personal preference there was no beating an SE535 plugged to a silent amplifier.
The SHA900 has been purchased about a week into owning a pair of SE846s, a small risk I took to accommodate the earphones (as there's a very small amount of reviews out there). To that point, my main setup and comparison point is a Schiit Vali 2 and Modi Multibit stack.
The impressions are made on a month-old SHA900 unit with 100+ play hours (on DAC mode) and EQ set to bypass/off.

Sound impressions:
There's two things I don't expect from high-end audio equipment: The first one is to make my "crappy" mid-fi headphones (Philips SHP9500 for instance*) sound anything worth talking about. The second, is any comments on the sound from my audio-Average Joe friends. So when I took the SHA900 to work, gave a listen to a colleague who's proud of his $100 Beats-knockoff, then a friend who has proclaimed he cannot hear a difference between his Skullcandy and my HE-400i, and both go "hey, this sounds a lot better!", I know this time I have something different in my hands.
The first thing that hits you with the SHA900 is the soundstage. It has a spacious, open sound, which makes focusing on instruments an easy task. Once you get acquainted with that, the second thing that hits you is the detail each individual in the music has. Listening to one element can be as enjoyable as hearing them all together. The third and final initial impression point is the decay of instruments, in particular the bass. Admittedly, the majority of the evaluation was done on a pair of Shure SE846s, which handle bass exceptionally well, but string instruments can also feel that they just fade on and on to absolute silence.
Speaking of, it should come as no surprise that this amp is silent, seeing as Shure advertises it to synergize very well with its in-ears. True enough, the SHA900 is easily the most silent amplifier I've ever heard (Fiio E12A included). Interestingly (and pleasantly surprising), this silent character applies both to background noise (hiss), as well as to the music itself, which feels to me as if there's more silence between instruments and during transitions. The cause for that is likely composed from various properties of the sound (width, I guess), and I find it as one of the strong points of this amp.

[A match made in heaven.]​

Macro is what I feel this unit excels in with flying remarks. I can drag on all day about punchy bass, full round vocals and airy guitar strings (all which are true), but it's the experience of listening to a full album, start to finish, which gives the final rating, and the SHA900 is capable of offering an unforgettable ride. I define a good audio experience as watching a good fantasy movie. Half-way through the film, you find yourself immersed in a different world with a different state of mind. In music, call it either ambience, musicality or just detail, for me, it is the sensation of being engulfed in the music, as if surrounded by or inside it. It may as well have started with the Shure SE535, having that numbing effect. The SE846 then further improved that sense of immersion, and the SHA900 has truly complemented and perfected it. Fed by a Schiit Modi Multibit, for the first in a very long time, I stopped caring to what I was listening through and simply enjoyed what I was hearing. This amplifier is the first piece of equipment I feel inclined to push the volume higher the more I listen to it, having the hearing loss warning etched into the unit being a testament to that.

(*) They sound great for their price, actually. Just passing a point. ^^

The DAC:
I've seen the unit's DAC being referred to from "old" to "crap", without any further explanation, so I wanted to dedicate a section of the review to address the subject. The internals of the SHA900 are largely a mystery. With a rather odd spec sheet, the amp section is possibly a unique Shure design (stemming from the KSA1500). The DAC however, is the Cirrus Logic CS4272, which came about in 2005 (hence the 'old' moniker).
Now, my experience with pricey DACs is rather limited (see comparisons section), so between the Schiit Modi Multibit and the original Fiio X5's TI PCM1792A DAC, the SHA900's DAC veers substantially more towards the prior. While obviously a function of implementation, the TI DAC sounds muddy and muffled in comparison to the CS4272. The Modi however does sound more detailed, spacious and more realistic - not as far as calling it night and day, but it's noticeable.
Either way, as part of a portable package, while not on the same level as the amp, I would still rate the DAC as very good and a well worthy match. It does not negatively affect the feeling of immersion I mentioned earlier, and while I do enjoy the SHA900 more when fed by the Modi, I don't get disappointed when I listen through the DAC on the go.
I am aware that there are very good portable, even multibit DACs out there (Chord Mojo the first that jumps to mind), but I've read reviews that go both ways, and as I haven't heard it myself, I can't comment on it. As for delta-sigma, I am curious about the recent AK4490s popping all over the place, and will update the review if I get my hands on a decent one.

Usability and user interface:
Operating the SHA900 is simple enough, with four buttons, three plugs and one small good-looking OLED screen. On the side, there is a slide-click lock button with the power button just above it, which also functions as the 'back/cancel' command in menus. It takes about half a second of pressing to fire the amp up (something a lot of companies could learn from), and the usual 3 seconds to turn it off. At the bottom there's the input switch, which sets the unit into either DAC or line-in mode. Needless to say, the unit can be charged while using either, and I don't notice any audio artifacts when charging and listening to music simultaneously. Finally, on top, there's the volume control knob that can be double-clicked to enter the menu, and single-clicked to navigate it.

[A simple button array.]​

On actually using the amp, I'm unfortunately greeted once more with the disease that plagues the complete majority of amps on the planet, which is the volume being too god damned loud. The amp has 25 volume levels, and even with Shure's in-ears (counter-intuitively), when plugged as a DAC, level 2 is enough to sound what I consider loud. In line-in mode, with input gain set to -20dB, 4 is still my personal limit. While a point can be made that there are many inefficient headphones out there (personal limit for the Hifiman HE-400i is level 5 on high gain), my expectations were for a much more fine-grained volume control. On the bright side, channels are absolutely balanced from the lowest volume point, and, as I mentioned, in line-in mode the input gain can be attenuated by -10dB and -20dB, which is a must when fed from a desktop DAC. Curiously, the spec sheet states for an adjustable gain range of -60 to +17 dB, which isn't accessible through the UI.
Speaking of which, the SHA900 sports the most simple and practical UI I came across in a portable. Double-click the wheel and you're greeted with an 'Equalizer', 'Audio' and 'Utilities' menu entries. The EQ admittedly, is my least accessed menu, other than occasionally using the 'Low Boost' preset. It does however allow a fine-grained, subtle (when needed) control of frequencies. Shure puts an emphasis on that feature of the amp, and when exploring and toying with the custom EQ presets, it's commendable, and easy to understand why: If you won't be using an external DAC, this is a very accomplished way of matching the sound to your headphones and personal preference. It is worth noting however, that while the EQ can be used when plugged via line-in, it will "default" the sound to the capabilities of the unit's ADC/DAC, which can degrade sound quality if your source is superior.
The next entry in the menu is 'Audio', a more frequently visited area for me. Output gain can be changed here - Low/High, both of which are dead silent - the mentioned line-in input attenuation that helps prevent clipping from high output sources, and an "analog RMS limiter", which function to the best of my understanding, is limiting volume spikes.

[The UI is simple and on-point.]​

The last menu, 'Utilities', controls technical aspects of the amp itself. The display's brightness and timeout can be adjusted, as well as its orientation (flipping the screen) and the volume pot's direction. Resetting and firmware updating is possible here as well, along with some info on battery health, its charge cycles and the unit's hardware info (which doesn't appear to mean much). One of my personal favourites is the ability to disable charging when the USB plug is connected, which appears to be a non-existent feature on a lot of DAC/amps and DAPs. This is particularly useful when plugged to a smartphone in DAC mode, as most battery-powered DACs I've tested so far, simply keep running (or charging) on power provided by the phone instead of using their own batteries, allowing about 3-4 hours of playback in the process.
In general, the usage experience feels like Shure knows their unit inside-out, and doesn't feel like there were two separate teams whose work was dumped in and merged at the last second (hi, Fiio).

(Limited) comparisons:
This is both fortunately and unfortunately my first foray into the 4-digit audio market, so equipment anywhere near the SHA900's price is rather scarce. Still, I figured it might interest people not into $100 jumps in their audio journey.
The first obvious comparison is to smartphones and laptops. In case somebody new to all this audio craziness is reading the review, with a decent pair of headphones/earphones from a respectable brand (Sennheiser, AKG, Shure, to name a few), the difference is day and night. In a quiet environment, you will hear a difference, and it will be enormous.
Unto some audiophile nonsense, the first first piece of equipment I have on hand is a Fiio X5/E12A portable stack. This is an IEM-targeted "great value/bang for your buck/competes with much higher priced" sort of thing, and one could argue that it can indeed compete with higher tier gear. The SHA900 however, sounds better in every single aspect, in both DAC and amp departments (except for the lack of hiss, which is pretty much the same). Long story short, it makes the Fiio stack sound like radio, answering the age-old question of whether double the money buys you "double the sound", which in this case, it actually does.
The second comparison is a more relevant one, and is a desktop Schiit Modi Multibit/Vali 2 stack. While I already mentioned the SHA900's DAC is a bit short of the Modi, in terms of the whole package, the Shure package presents a smoother, slightly more refined and more immersive listening experience. The cause for that, of course, is the Vali - while no slouch by any measurement, it doesn't quite portray the music in an effortless, airy presentation as the SHA900. While the various tubes help shape the sound to match particular genres and listening preferences, compared to the Vali, I find the SHA900 to be the jack of all trades - having the detail and soundstage of the stock 6BZ7, and the smooth, slightly warm lows of an EH 6CG7. Admittedly, not a fair comparison considering the price difference, but the fact a portable amplifier is able to outperform a very good desktop setup is worth mentioning.

Build, accessories and miscellaneous:
On the subject of construction, at first glance, it's a mixed bag: Modern construction trends dictate glass and metal in every millimeter of the product to give you that premium weight in your hand, yet the SHA900 is light, the buttons are plastic and none of them is particularly fixed to the unit (a bit wobbly). Once you turn the unit on however, the impression improves with the sharp-looking OLED screen, which doesn't suffer from the common washed under-saturated colours (all 6 of them ^^') of cheaper counterparts. Finally, examining the unit from up-close reveals an excellent build - there's absolutely zero mismatch between the layers, nothing popping out, and other than the feel of the buttons, the unit both looks and touches a match for its price.

[Both materials and construction are great.]​

The accessories of the SHA900 are, other than the cables, largely useless, which from experience appears to be a Shure thing. I can understand the rubber bands, but top to bottom, the cleaning cloth has a silky texture that instead of wiping your fingerprints, just smears them (unlike cotton); the airline adapter is useful if you're flying North Korean (i.e., deprecated); the volume attenuer sounds god-awful in any scenario I tried it; the 6.3mm adapter is very situational - if anything, I would've prefered a 6.3mm to 3.5mm cable adapter, as these plugs are more common with high-end headphones. Also, including a wall charger with a swappable plug is nice, except that there aren't any additional plugs provided. But the icing on the Should-Have cake is the lack of a case of any sort. At first I thought there has been a mistake at the shop I bought the unit from, as the KSE1500 set has one, but as it turns out, $1k is not enough to include one with the package. The SHA900 isn't exactly brittle, and personally I would have sufficed with faux-leather or rubber, but I still feel it's a cheap move on Shure's side.

Concluding remarks:
Welp, this has been my longest review by far, on my most expensive piece of equipment so far (which I hope remains that way). To sum this up in a paragraph, the Shure SHA900 is a subjectively exceptional sounding portable amplifier with an optional good-sounding DAC, has an accomplished UI with well-thought-of features and commendable attention to detail, and has an excellent construction to pack it all up. On a grayer side, one could ask for finer-grained volume control and the accessories are rather lacking.
More than once the subject of price has been a point of debate over this unit. All things considered, this is unavoidably a luxury multi-buck product: It doesn't sport a "great value" moniker, nor crams as many functions and features as possible into a single box. But in my opinion and personal experience, it doesn't need to, either. Instead, it achieves precisely what it's advertised to do: It costs close to a grand, feels like it, sounds like it, is built to standard, and ultimately, it's a product well worthy of the brand's reputation.